The long-term goal of this project is to construct an in vitro transcription system for the ribosomal RNA transcription unit and to purify and characterize as to molecular mechanism the components of this system. The present project involves the identification of the start site for the transcription unit, the development of the conditions for assembly of the in vitro system from recombinant DNA, purified RNA polymerase I and crude cellular extracts (S100 extracts) containing ancillary proteins needed for faithful transcription.